The History of Enterovirus A71 Outbreaks and Molecular Epidemiology in the Asia-Pacific Region

Enterovirus A71 (EV-A71) is one of the common causative pathogens for hand foot and mouth disease (HFMD) affecting young children. HFMD outbreak can result in a substantial pediatric hospitalization and burden the healthcare services, especially in less-developed countries. Since the initial epidemic of predominantly EV-A71 in California in 1969, the high prevalence of HFMD in the Asia-pacific region and elsewhere around the world represents a significant morbidity in this age group. With the advent of rapid and accurate diagnostic tools, there has been a dramatic increase in the number of laboratory-confirmed EV-A71 infection over the past two decades. The population, cultural, and socioeconomic diversity among countries in the Asia-Pacific region all influence the transmission and morbidity associated with HFMD. This review summarizes the current state of epidemiology of EV-A71 in Asia-Pacific countries based on the most recent epidemiological data and available information on the prevalence and disease burden. This knowledge is important in guiding the prevention, control and future research on vaccine development of this highly contagious disease of significant socioeconomic implications in public health.

Cell surface vimentin is an attachment receptor for enterovirus 71

Enterovirus 71 (EV71) is a highly transmissible pathogenic agent that causes severe central nervous system diseases in infected infants and young children. Here, we reported that EV71 VP1 protein could bind to vimentin intermediate filaments expressed on the host cell surface. Soluble vimentin or an antibody against vimentin could inhibit the binding of EV71 to host cells. Accompanied with the reduction of vimentin expression on the cell surface, the binding of EV71 to cells was remarkably decreased. Further evidence showed that the N terminus of vimentin is responsible for the interaction between EV71 and vimentin. These results indicated that vimentin on the host cell surface may serve as an attachment site that mediated the initial binding and subsequently increased the infectivity of EV71.

IMPORTANCE

This study delivers important findings on the roles of vimentin filaments in relation to EV71 infection and provides information that not only improves our understanding of EV71 pathogenesis but also presents us with potentially new strategies for the treatment of diseases caused by EV71 infections.

Transgenic mouse model for the study of enterovirus 71 neuropathogenesis

Enterovirus 71 (EV71) typically causes mild hand-foot-and-mouth disease in children, but it can also cause severe neurological disease. Recently, epidemic outbreaks of EV71 with significant mortality have been reported in the Asia-Pacific region, and EV71 infection has become a serious public health concern worldwide. However, there is little information available concerning EV71 neuropathogenesis, and no vaccines or anti-EV71 drugs have been developed. Previous studies of this disease have used monkeys and neonatal mice that are susceptible to some EV71 strains as models. The monkey model is problematic for ethical and economical reasons, and mice that are more than a few weeks old lose their susceptibility to EV71. Thus, the development of an appropriate small animal model would greatly contribute to the study of this disease. Mice lack EV71 susceptibility due to the absence of a receptor for this virus. Previously, we identified the human scavenger receptor class B, member 2 (hSCARB2) as a cellular receptor for EV71. In the current study, we generated a transgenic (Tg) mouse expressing hSCARB2 with an expression profile similar to that in humans. Tg mice infected with EV71 exhibited ataxia, paralysis, and death. The most severely affected cells were neurons in the spinal cord, brainstem, cerebellum, hypothalamus, thalamus, and cerebrum. The pathological features in these Tg mice were generally similar to those of EV71 encephalomyelitis in humans and experimentally infected monkeys. These results suggest that this Tg mouse could represent a useful animal model for the study of EV71 infection.

Human SCARB2-dependent infection by coxsackievirus A7, A14, and A16 and enterovirus 71

Human enterovirus species A (HEV-A) consists of at least 16 members of different serotypes that are known to be the causative agents of hand, foot, and mouth disease (HFMD), herpangina, and other diseases, such as respiratory disease and polio-like flaccid paralysis. Enterovirus 71 (EV71) and coxsackievirus A16 (CVA16) are the major causative agents of HFMD. CVA5, CVA6, CVA10, and CVA12 mainly cause herpangina or are occasionally involved with sporadic cases of HFMD. We have previously shown that human scavenger receptor class B, member 2 (SCARB2) is a cellular receptor for EV71 and CVA16. Using a large number of clinical isolates of HEV-A, we explored whether all clinical isolates of EV71 and other serotypes of HEV-A infected cells via SCARB2. We tested this possibility by infecting L-SCARB2 cells, which are L929 cells expressing human SCARB2, by infecting human RD cells that had been treated with small interfering RNAs for SCARB2 and by directly binding the viruses to a soluble SCARB2 protein. We showed that all 162 clinical isolates of EV71 propagated in L-SCARB2 cells, suggesting that SCARB2 is the critical receptor common to all EV71 strains. In addition, CVA7, CVA14, and CVA16, which are most closely related to each other, also utilized SCARB2 for infection. EV71, CVA14, and CVA16 are highly associated with HFMD, and EV71 and CVA7 are occasionally associated with neurological diseases, suggesting that SCARB2 plays important roles in the development of these diseases. In contrast, another group of viruses, such as CVA2, CVA3, CVA4, CVA5, CVA6, CVA8, CVA10, and CVA12, which are relatively distant from the EV71 group, is associated mainly with herpangina. None of these clinical isolates infected via the SCARB2-dependent pathway. HEV-A viruses can be divided into at least two groups depending on the use of SCARB2, and the receptor usage plays an important role in developing the specific diseases for each group.

Hand foot and mouth disease due to enterovirus 71 in Malaysia

Hand foot and mouth disease is a febrile sickness complex characterized by cutaneous eruption (exanthem) on the palms and soles with simultaneous occurrence of muco-cutanous vesiculo-ulcerative lesions (enanthem) affecting the mouth. The illness is caused by a number of enteroviruses with coxsackievirus A16 and enterovirus 71 as the main causative agents. Human enterovirus 71 (EV71) belongs to the species Human enterovirus A under the genus Enterovirus within the family Picornaviridae. EV71 has been associated with an array of clinical diseases including hand foot and mouth disease (HFMD), aseptic meningitis, encephalitis and poliomyelitis-like acute flaccid paralysis. A large outbreak of HFMD due to highly neurovirulent EV71 emerged in Malaysia in 1997, and caused 41 deaths amongst young children. In late 2000, a recurrence of an outbreak of HFMD occurred in Malaysia with 8 fatalities in peninsular Malaysia. Outbreak of HFMD due to EV71 recurred in 2003 with an unknown number of cases and mortalities. A similar outbreak of HFMD with 2 recorded deaths in young children occurred in peninsular Malaysia in late 2005 and this was followed by a larger outbreak in Sarawak (Malaysian Borneo) with 6 reported fatalities in the early part of 2006. The current on-going outbreak of HFMD started in peninsular Malaysia in epidemiological week 12 of 2010. As with other HFMD outbreaks in Malaysia, both EV71 and CA16 were the main aetiological viruses isolated. In similarity with the HFMD outbreak in 2005, the isolation of CA16 preceded the appearance of EV71. Based on the VP1 gene nucleotide sequences, 4 sub-genogroups of EV71 (C1, C2, B3 and B4) co-circulated and caused the outbreak of hand, foot and mouth disease in peninsular Malaysia in 1997. Two sub-genogroups (C1 and B4) were noted to cause the outbreak in 2000 in both peninsular Malaysia and Sarawak. EV71 of sub-genogroup B5 with smaller contribution from sub-genogroup C1 caused the outbreak in 2003. In the 2005 outbreak, besides the EV71 strains of sub-genogroup C1, EV71 strains belonging to sub-genogroup B5 were isolated but formed a cluster which was distinct from the EV71 strains from the sub-genogroup B5 isolated in 2003. The four EV71 strains isolated from clinical specimens of patients with hand, foot and mouth disease in the Sarawak outbreak in early 2006 also belonged to sub-genogroup B5. Phylogenetic analysis of the VP1 gene suggests that the EV71 strains causing the outbreak in Sarawak could have originated from peninsular Malaysia. Epidemiological and molecular data since 1997 show the recurrence of HFMD due to EV71 in Malaysia every 2 to 4 years. In each of the past outbreaks, more than one sub-genogroup of the virus co-circulate.

Antiviral effect of epigallocatechin gallate on enterovirus 71

Oxidative stress is known to be a determinant of a host's susceptibility to pathogens. Natural compounds with antioxidant activity may provide a preventive measure against infection. Tea polyphenols were evaluated for their ability to inhibit enterovirus 71 (EV71) replication in Vero cell culture. Among the polyphenolic compounds tested, epigallocatechin gallate (EGCG) and gallocatechin gallate (GCG) potently inhibited replication of EV71. EGCG and GCG reduced the titer of infectious progeny virus by 95%. Quantitative RT-PCR analysis also revealed that EGCG suppressed replication of genomic RNA. It was accompanied by an increased cytoprotective effect. EGCG and GCG caused 5-fold increase in the viability of EV71-infected cells. The viral inhibitory effect correlated well with the antioxidant capacity of polyphenol. Mechanistically, EV71 infection led to increased oxidative stress, as shown by increased dichlorofluorescein and MitoSOX Red fluorescence. Upon EGCG treatment, reactive oxygen species (ROS) generation was significantly reduced. Consistent with this, EV71 replication was enhanced in glucose-6-phosphate dehydrogenase deficient cells, and such enhancement was largely reversed by EGCG. These findings suggest that EGCG may suppress viral replication via modulation of cellular redox milieu.

Co-circulation and genomic recombination of coxsackievirus A16 and enterovirus 71 during a large outbreak of hand, foot, and mouth disease in Central China

A total of 1844 patients with hand, foot, and mouth disease (HFMD), most of them were children of age 1–3-year-old, in Central China were hospitalized from 2011 to 2012. Among them, 422 were infected with coxsackievirus A16 (CVA16), 334 were infected with enterovirus 71 (EV71), 38 were co-infected with EV71 and CVA16, and 35 were infected with other enteroviruses. Molecular epidemiology analysis revealed that EV71 and CVA16 were detected year-round, but EV71 circulated mainly in July and CVA16 circulated predominantly in November, and incidence of HFMD was reduced in January and February and increased in March. Clinical data showed that hyperglycemia and neurologic complications were significantly higher in EV71-infected patients, while upper respiratory tract infection and C-reactive protein were significantly higher in CVA16-associated patients. 124 EV71 and 80 CVA16 strains were isolated, among them 56 and 68 EV71 strains were C4a and C4b, while 25 and 55 CVA16 strains were B1a and B1b, respectively. Similarity plots and bootscan analyses based on entire genomic sequences revealed that the three C4a sub-genotype EV71 strains were recombinant with C4b sub-genotype EV71 in 2B–2C region, and the three CVA16 strains were recombinant with EV71 in 2A–2B region. Thus, CVA16 and EV71 were the major causative agents in a large HFMD outbreak in Central China. HFMD incidence was high for children among household contact and was detected year-round, but outbreak was seasonal dependent. CVA16 B1b and EV71 C4b reemerged and caused a large epidemic in China after a quiet period of many years. Moreover, EV71 and CVA16 were co-circulated during the outbreak, which may have contributed to the genomic recombination between the pathogens. It should gain more attention as there may be an upward trend in co-circulation of the two pathogens globally and the new role recombination plays in the emergence of new enterovirus variants.

Systemic Therapy of Malignant Human Melanoma Tumors by a Common Cold-Producing Enterovirus, Coxsackievirus A21

PURPOSE

The incidence of malignant melanoma continues to increase worldwide; however, treatment of metastatic melanoma remains unsatisfactory, and there is an urgent need for development of effective targeted therapeutics. A potential biological target on the surface of malignant melanoma cells is the up-regulated expression of intercellular adhesion molecule (ICAM)-1 and decay-accelerating factor (DAF), relative to surrounding benign tissue. Coxsackievirus A21 (a common cold virus) targets and destroys susceptible cells via specific viral capsid interactions with surface-expressed virus receptors comprising ICAM-1 and DAF.

EXPERIMENTAL DESIGN

The oncolytic capacity of a genetically unmodified wild-type common cold-producing human enterovirus (Coxsackievirus A21, CAV21) was assessed against in vitro cultures and in vivo xenografts of malignant human melanoma cells.

RESULTS

In vitro studies established that human melanoma cells endogenously express elevated levels of ICAM-1/DAF and were highly susceptible to rapid viral oncolysis by CAV21 infection, whereas ICAM-1/DAF-expressing peripheral blood lymphocytes were refractile to infection. In vivo studies revealed that the tumor burden of nonobese diabetic severe combined immunodeficient mice bearing multiple s.c. melanoma xenografts was rapidly reduced by oncolysis mediated by a single administration of CAV21. The antitumor activity of CAV21 was characterized by highly efficient systemic spread of progeny CAV21, with oncolysis of tumors also occurring at sites distant to the primary site of viral administration.

CONCLUSIONS

Overall, the findings presented herein demonstrate an important proof of principle using administration of replication-competent CAV21 as a potential biological oncolytic agent in the control of human metastatic melanoma.

Functional comparison of SCARB2 and PSGL1 as receptors for enterovirus 71

Human scavenger receptor class B, member 2 (SCARB2), and P-selectin glycoprotein ligand-1 (PSGL1) have been identified to be the cellular receptors for enterovirus 71 (EV71). We compared the EV71 infection efficiencies of mouse L cells that expressed SCARB2 (L-SCARB2) and PSGL1 (L-PSGL1) and the abilities of SCARB2 and PSGL1 to bind to the virus. L-SCARB2 cells bound a reduced amount of EV71 compared to L-PSGL1 cells. However, EV71 could infect L-SCARB2 cells more efficiently than L-PSGL1 cells. The results suggested that the difference in the binding capacities of the two receptors was not the sole determinant of the infection efficiency and that SCARB2 plays an essential role after attaching to virions. Therefore, we examined the viral entry into L-SCARB2 cells and L-PSGL1 cells by immunofluorescence microscopy. In both cells, we detected internalized EV71 virions that colocalized with an early endosome marker. We then performed a sucrose density gradient centrifugation analysis to evaluate viral uncoating. After incubating the EV71 virion with L-SCARB2 cells or soluble SCARB2 under acidic conditions below pH 6.0, we observed that part of the native virion was converted into an empty capsid that lacked both genomic RNA and VP4 capsid proteins. The results suggested that the uncoating of EV71 requires both SCARB2 and an acidic environment and occurs after the internalization of the virus-receptor complex into endosomes. However, the empty capsid formation was not observed after incubation with L-PSGL1 cells or soluble PSGL1 under any of the tested pH conditions. These results indicated that SCARB2 is capable of viral binding, viral internalization, and viral uncoating and that the low infection efficiency of L-PSGL1 cells is due to the inability of PSGL1 to induce viral uncoating. The characterization of SCARB2 as an uncoating receptor greatly contributes to the understanding of the early steps of EV71 infection.

Enterovirus 71 binding to PSGL-1 on leukocytes: VP1-145 acts as a molecular switch to control receptor interaction

Some strains of enterovirus 71 (EV71), but not others, infect leukocytes by binding to a specific receptor molecule: the P-selectin glycoprotein ligand-1 (PSGL-1). We find that a single amino acid residue within the capsid protein VP1 determines whether EV71 binds to PSGL-1. Examination of capsid sequences of representative EV71 strains revealed that the PSGL-1-binding viruses had either a G or a Q at residue 145 within the capsid protein VP1 (VP1-145G or Q), whereas PSGL-1-nonbinding viruses had VP1-145E. Using site-directed mutagenesis we found that PSGL-1-binding strains lost their capacity to bind when VP1-145G/Q was replaced by E; conversely, nonbinding strains gained the capacity to bind PSGL-1 when VP1-145E was replaced with either G or Q. Viruses with G/Q at VP1-145 productively infected a leukocyte cell line, Jurkat T-cells, whereas viruses with E at this position did not. We previously reported that EV71 binds to the N-terminal region of PSGL-1, and that binding depends on sulfated tyrosine residues within this region. We speculated that binding depends on interaction between negatively charged sulfate groups and positively charged basic residues in the virus capsid. VP1-145 on the virus surface is in close proximity to conserved lysine residues at VP1-242 and VP1-244. Comparison of recently published crystal structures of EV71 isolates with either Q or E at VP1-145 revealed that VP1-145 controls the orientation of the lysine side-chain of VP1-244: with VP1-145Q the lysine side chain faces outward, but with VP1-145E, the lysine side chain is turned toward the virus surface. Mutation of VP1-244 abolished virus binding to PSGL-1, and mutation of VP1-242 greatly reduced binding. We propose that conserved lysine residues on the virus surface are responsible for interaction with sulfated tyrosine residues at the PSGL-1 N-terminus, and that VP1-145 acts as a switch, controlling PSGL-1 binding by modulating the exposure of VP1-244K.

Enterovirus 71 (EV71) commonly causes mild febrile illness in children (hand, foot, and mouth disease), but some patients suffer severe neurologic disease and death. Recent outbreaks in the Asia-Pacific region have caused thousands of deaths, making EV71 a major public health concern. Some EV71 strains bind to P-selectin glycoprotein ligand-1 (PSGL-1) and infect immune cells, but others do not. We previously found that EV71 binds the PSGL-1 N-terminus, and that binding depends on tyrosine sulfation of the N-terminus, but the viral factors that control interaction with PSGL-1 have not been identified. In our present work we present evidence that a single amino acid, residue 145 of the viral capsid protein (VP1-145), determines whether a virus binds or does not bind PSGL-1, and that it functions by influencing the orientation of a nearby lysine residue (VP1-244) on the virus surface. We propose that VP1-145 controls virus tropism by changing the accessibility of the positively-charged lysine side chain of VP1-244 to the negatively charged, sulfated N-terminus of PSGL-1. Our results shed new light on virus-receptor interaction, and EV71 tropism for PSGL-1-expressing leukocytes.

Molecular mechanism of SCARB2-mediated attachment and uncoating of EV71

Unlike the well-established picture for the entry of enveloped viruses, the mechanism of cellular entry of non-enveloped eukaryotic viruses remains largely mysterious. Picornaviruses are representative models for such viruses, and initiate this entry process by their functional receptors. Here we present the structural and functional studies of SCARB2, a functional receptor of the important human enterovirus 71 (EV71). SCARB2 is responsible for attachment as well as uncoating of EV71. Differences in the structures of SCARB2 under neutral and acidic conditions reveal that SCARB2 undergoes a pivotal pH-dependent conformational change which opens a lipid-transfer tunnel to mediate the expulsion of a hydrophobic pocket factor from the virion, a pre-requisite for uncoating. We have also identified the key residues essential for attachment to SCARB2, identifying the canyon region of EV71 as mediating the receptor interaction. Together these results provide a clear understanding of cellular attachment and initiation of uncoating for enteroviruses.

Type III interferon signaling restricts enterovirus 71 infection of goblet cells

Recent worldwide outbreaks of enterovirus 71 (EV71) have caused major epidemics of hand, foot, and mouth disease with severe neurological complications, including acute flaccid paralysis. EV71 is transmitted by the enteral route, but little is known about the mechanisms it uses to cross the human gastrointestinal tract. Using primary human intestinal epithelial monolayers, we show that EV71 infects the epithelium from the apical surface, where it preferentially infects goblet cells. We found that EV71 infection did not alter epithelial barrier function but did reduce the expression of goblet cell-derived mucins, suggesting that it alters goblet cell function. We also show that the intestinal epithelium responds to EV71 infection through the selective induction of type III interferons (IFNs), which restrict EV71 replication. Collectively, these findings define the early events associated with EV71 infections of the human intestinal epithelium and show that host IFN signaling controls replication in an IFN-specific manner.

Identification of site-specific adaptations conferring increased neural cell tropism during human enterovirus 71 infection

Enterovirus 71 (EV71) is one of the most virulent enteroviruses, but the specific molecular features that enhance its ability to disseminate in humans remain unknown. We analyzed the genomic features of EV71 in an immunocompromised host with disseminated disease according to the different sites of infection. Comparison of five full-length genomes sequenced directly from respiratory, gastrointestinal, nervous system, and blood specimens revealed three nucleotide changes that occurred within a five-day period: a non-conservative amino acid change in VP1 located within the BC loop (L97R), a region considered as an immunogenic site and possibly important in poliovirus host adaptation; a conservative amino acid substitution in protein 2B (A38V); and a silent mutation in protein 3D (L175). Infectious clones were constructed using both BrCr (lineage A) and the clinical strain (lineage C) backgrounds containing either one or both non-synonymous mutations. In vitro cell tropism and competition assays revealed that the VP1₉₇ Leu to Arg substitution within the BC loop conferred a replicative advantage in SH-SY5Y cells of neuroblastoma origin. Interestingly, this mutation was frequently associated in vitro with a second non-conservative mutation (E167G or E167A) in the VP1 EF loop in neuroblastoma cells. Comparative models of these EV71 VP1 variants were built to determine how the substitutions might affect VP1 structure and/or interactions with host cells and suggest that, while no significant structural changes were observed, the substitutions may alter interactions with host cell receptors. Taken together, our results show that the VP1 BC loop region of EV71 plays a critical role in cell tropism independent of EV71 lineage and, thus, may have contributed to dissemination and neurotropism in the immunocompromised patient.

Rupintrivir is a promising candidate for treating severe cases of enterovirus-71 infection: evaluation of antiviral efficacy in a murine infection model

Enterovirus-71 (EV71) infections can cause life-threatening diseases with neurological symptoms. Currently, no direct targeting antivirals are available to combat severe EV71 infection. Rupintrivir (AG7088) is a compound originally designed for Rhinovirus 3C protease. Previous computational analyses by us and crystallography studies by others suggested that rupintrivir is also a high affinity inhibitor to EV71 3C. Thus, we aimed to further evaluate its anti-EV71 activity in vivo at clinically acceptable doses. It was observed that administration of rupintrivir in suckling mice largely protected them from limb paralysis and dramatically improved survival (38.5% DMSO vs. 90.9% at 0.1mg/kg, p=0.006). Histological, immunohistochemical and quantitative RT-PCR analyses confirmed that rupintrivir profoundly alleviated virus induced necrotizing myositis, suppressed viral RNA and blocked EV71 VP1 expression in various tissues. In conclusion, we established that rupintrivir can strongly contain the spread of EV71 infection in vivo at a clinically acceptable dose (as low as 0.1mg/kg). As its safety has been fully tested in previous clinical trials, rupintrivir is suitable for immediate evaluation of potential benefits in EV71-infected individuals with life-threatening neurological symptoms.

Enterovirus 71 3C inhibits cytokine expression through cleavage of the TAK1/TAB1/TAB2/TAB3 complex

Enterovirus 71 (EV71) causes hand, foot, and mouth disease in young children and infants. Severe infection with EV71 can lead to various neurological complications or fatal diseases. However, the mechanism of EV71 pathogenesis is poorly understood. Emerging evidence suggests that EV71 modulates type I interferon (IFN) and cytokine responses. Here, we show that EV71 disables components of the TAB2 complex through the 3C protein. When expressed in mammalian cells, EV71 3C interacts with TAB2 and TAK1, which inhibits NF-κB activation. Furthermore, 3C mediates cleavage of TAB2 and its partners, which requires the protease activity. H40D or C147S substitution in the 3C active sites abolishes its activity, whereas R84Q or V154S substitution in the RNA binding domain has no effect. The 3C protein targets TAB2 at Q113-S114, TAK1 at Q360-S361, TAB1 both at Q414-G415 and Q451-S452, and TAB3 at Q173-G174 and Q343-G344. Importantly, overexpression of TAB2 inhibits EV71 replication, whereas addition of cleaved fragments has no effect. Thus, an equilibrium between the TAB2 complex and EV71 3C represents a control point of viral infection. These results suggest that TAK1/TAB1/TAB2/TAB3 cleavage mediated by EV71 may be a mechanism to interfere with inflammatory responses.

IMPORTANCE

The TAK1 complex plays a critical role in the activation of NF-κB and cytokine production. However, little is known about its connection to enterovirus 71 (EV71). We demonstrate that EV71 3C suppresses cytokine expression via cleavage of the TAK1 complex proteins. EV71 3C interacts with TAB2 and TAK1. Furthermore, overexpression of TAB2 inhibits EV71 replication, whereas addition of cleaved fragment has no effect. These results suggest that the interplay of EV71 and the TAK1 complex influences the outcome of viral infection.

Identification of a human SCARB2 region that is important for enterovirus 71 binding and infection

We previously identified human scavenger receptor class B, member 2 (SCARB2), as a cellular receptor for enterovirus 71 (EV71). Expression of human SCARB2 (hSCARB2) permitted mouse L929 cells to efficiently bind to virions and to produce both viral proteins and progeny viruses upon EV71 infection. Mouse Scarb2 (mScarb2) exhibited 85.8% amino acid identity and 99.9% similarity to hSCARB2. The expression of mScarb2 in L929 cells conferred partial susceptibility. Very few virions bound to mScarb2-expressing cells. The viral titer in L929 cells expressing mScarb2 was approximately 40- to 100-fold lower than that in L929 cells expressing hSCARB2. Using hSCARB2-mScarb2 chimeric mutants, we attempted to map the region that was important for efficient EV71 infection. L929 cells expressing chimeras that carried amino acids 142 to 204 from the human sequence were susceptible to EV71, while chimeras that carried the mouse sequence in this region were not. Moreover, this region was also critical for binding to virions. The determination of this region in hSCARB2 that is important for EV71 binding and infection greatly contributes to the understanding of virus-receptor interactions. Further studies will clarify the early steps of EV71 infection.

Prohibitin plays a critical role in Enterovirus 71 neuropathogenesis

A close relative of poliovirus, enterovirus 71 (EV71) is regarded as an important neurotropic virus of serious public health concern. EV71 causes Hand, Foot and Mouth Disease and has been associated with neurological complications in young children. Our limited understanding of the mechanisms involved in its neuropathogenesis has hampered the development of effective therapeutic options. Here, using a two-dimensional proteomics approach combined with mass spectrometry, we have identified a unique panel of host proteins that were differentially and dynamically modulated during EV71 infection of motor-neuron NSC-34 cells, which are found at the neuromuscular junctions where EV71 is believed to enter the central nervous system. Meta-analysis with previously published proteomics studies in neuroblastoma or muscle cell lines revealed minimal overlapping which suggests unique host-pathogen interactions in NSC-34 cells. Among the candidate proteins, we focused our attention on prohibitin (PHB), a protein that is involved in multiple cellular functions and the target of anti-cancer drug Rocaglamide (Roc-A). We demonstrated that cell surface-expressed PHB is involved in EV71 entry into neuronal cells specifically, while membrane-bound mitochondrial PHB associates with the virus replication complex and facilitates viral replication. Furthermore, Roc-A treatment of EV71-infected neuronal cells reduced significantly virus yields. However, the inhibitory effect of Roc-A on PHB in NSC-34 cells was not through blocking the CRAF/MEK/ERK pathway as previously reported. Instead, Roc-A treated NSC-34 cells had lower mitochondria-associated PHB and lower ATP levels that correlated with impaired mitochondria integrity. In vivo, EV71-infected mice treated with Roc-A survived longer than the vehicle-treated animals and had significantly lower virus loads in their spinal cord and brain, whereas virus titers in their limb muscles were comparable to controls. Together, this study uncovers PHB as the first host factor that is specifically involved in EV71 neuropathogenesis and a potential drug target to limit neurological complications.

Anti-enterovirus 71 effects of chrysin and its phosphate ester

Enterovirus 71 (EV71) can cause severe disease and even lead to death in children, and an effective antiviral drug is currently unavailable. The anti-EV71 effect of chrysin (5,7-dihydroxyflavone), a natural flavonoid commonly found in many plants, was tested in this report. By using the predicting program Autodock 4.0 and an in vitro protease inhibition assay, we found that chrysin could suppress viral 3Cpro activity. Replication of viral RNA and production of viral capsid protein and the infectious virion were strongly inhibited by chrysin, without noticeable cytotoxicity. Cytopathic effects on cells were also prevented. Diisopropyl chrysin-7-yl phosphate (CPI), the phosphate ester for chrysin, was generated through a simplified Atheron-Todd reaction to achieve stronger anti-viral activity. CPI was also able to bind with and inhibit viral 3Cpro activity in vitro. As expected, CPI demonstrated more potent antiviral activity against EV71.

Resveratrol inhibits enterovirus 71 replication and pro-inflammatory cytokine secretion in rhabdosarcoma cells through blocking IKKs/NF-κB signaling pathway

Polydatin and resveratrol, as major active components in Polygonum cuspidatum, have anti-inflammatory, antioxidant and antitumor functions. However, the effect and mechanism of polydatin and resveratrol on enterovirus 71 (EV71) have not been reported. In this study, resveratrol revealed strong antiviral activity on EV71, while polydatin had weak effect. Neither polydatin nor resveratrol exhibited influence on viral attachment. Resveratrol could effectively inhibit the synthesis of EV71/VP1 and the phosphorylation of IKKα, IKKβ, IKKγ, IKBα, NF-κB p50 and NF-κB p65, respectively. Meanwhile, the remarkably increased secretion of IL-6 and TNF-α in EV71-infected rhabdosarcoma (RD) cells could be blocked by resveratrol. These results demonstrated that resveratrol inhibited EV71 replication and cytokine secretion in EV71-infected RD cells through blocking IKKs/NF-κB signaling pathway. Thus, resveratrol may have potent antiviral effect on EV71 infection.

Epidemiological Characteristics and Spatial-Temporal Clusters of Hand, Foot, and Mouth Disease in Zhejiang Province, China, 2008-2012

Hand, foot and mouth disease (HFMD) is one of the major public health concerns in China. Being the province with high incidence rates of HFMD, the epidemiological features and the spatial-temporal patterns of Zhejiang Province were still unknown. The objective of this study was to investigate the epidemiological characteristics and the high-incidence clusters, as well as explore some potential risk factors. The surveillance data of HFMD during 2008-2012 were collected from the communicable disease surveillance network system of Zhejiang Provincial Center for Disease Control and Prevention. The distributions of age, gender, occupation, season, region, pathogen's serotype and disease severity were analyzed to describe the epidemiological features of HFMD in Zhejiang Province. Seroprevalence survey for human enterovirus 71 (EV71) in 549 healthy children of Zhejiang Province was also performed, as well as 27 seroprevalence publications between 1997 and 2015 were summarized. The spatial-temporal methods were performed to explore the clusters at county level. Furthermore, pathogens' serotypes such as EV71 and coxsackievirus A16 (Cox A16) and meteorological factors were analyzed to explore the potential factors associated with the clusters. A total of 454,339 HFMD cases were reported in Zhejiang Province during 2008-2012, including 1688 (0.37%) severe cases. The annual average incidence rate was 172.98 per 100,000 (ranged from 72.61 to 270.04). The male-to-female ratio for mild cases was around 1.64:1, and up to 1.87:1 for severe cases. Of the total cases, children aged under three years old and under five years old accounted for almost 60% and 90%, respectively. Among all enteroviruses, the predominant serotype was EV71 (49.70%), followed by Cox A16 (26.05%) and other enteroviruses (24.24%) for mild cases. In severe cases, EV71 (82.85%) was the major causative agent. EV71 seroprevalence survey in healthy children confirmed that occult infection was common in children. Furthermore, literature summary for 26 seroprevalence studies during 1997-2015 confirmed that 0-5 years group showed lowest level of EV71 seroprevalence (29.1% on average) compared to the elder children (6-10 years group: 54.6%; 11-20 years group: 61.8%). Global positive spatial autocorrelation patterns (Moran's Is>0.25, P<0.05) were discovered not only for mild cases but also for severe cases, and local positive spatial autocorrelation patterns were revealed for counties from the eastern coastal and southern regions. The retrospective space-time cluster analysis also confirmed these patterns. Risk factors analyses implied that more EV71 and less sunshine were associated with the clusters of HFMD in Zhejiang Province. Our study confirmed that Zhejiang Province was one of the highly epidemic provinces in China and that the epidemiological characteristics of HFMD were similar to other provinces. Occult infection in elder children and adults was one of the important reasons why most HFMD cases were children aged under-five. Combining the results of spatial autocorrelation analysis and the space-time cluster analysis, the major spatial-temporal clusters were from the eastern coastal and southern regions. The distribution of pathogens' serotypes and the level of sunshine could be risk factors for, and serve as an early warning of, the outbreak of HFMD in Zhejiang Province.

Tonsillar crypt epithelium is an important extra-central nervous system site for viral replication in EV71 encephalomyelitis

Enterovirus 71 (EV71; family Picornaviridae, species human Enterovirus A) usually causes hand, foot, and mouth disease, which may rarely be complicated by fatal encephalomyelitis. We investigated extra-central nervous system (extra-CNS) tissues capable of supporting EV71 infection and replication, and have correlated tissue infection with expression of putative viral entry receptors, scavenger receptor B2 (SCARB2), and P-selectin glycoprotein ligand-1 (PSGL-1). Formalin-fixed, paraffin-embedded CNS and extra-CNS tissues from seven autopsy cases were examined by IHC and in situ hybridization to evaluate viral antigens and RNA. Viral receptors were identified with IHC. In all seven cases, the CNS showed stereotypical distribution of inflammation and neuronal localization of viral antigens and RNA, confirming the clinical diagnosis of EV71 encephalomyelitis. In six cases in which tonsillar tissues were available, viral antigens and/or RNA were localized to squamous epithelium lining the tonsillar crypts. Tissues from the gastrointestinal tract, pancreas, mesenteric nodes, spleen, and skin were all negative for viral antigens/RNA. Our novel findings strongly suggest that tonsillar crypt squamous epithelium supports active viral replication and represents an important source of viral shedding that facilitates person-to-person transmission by both the fecal-oral or oral-oral routes. It may also be a portal for viral entry. A correlation between viral infection and SCARB2 expression appears to be more significant than for PSGL-1 expression.

Caveolar endocytosis is required for human PSGL-1-mediated enterovirus 71 infection

Enterovirus 71 (EV71) causes hand, foot, and mouth disease and severe neurological disorders in children. Human scavenger receptor class B member 2 (hSCARB2) and P-selectin glycoprotein ligand-1 (PSGL-1) are identified as receptors for EV71. The underling mechanism of PSGL-1-mediated EV71 entry remains unclear. The endocytosis required for EV71 entry were investigated in Jurkat T and mouse L929 cells constitutively expressing human PSGL-1 (PSGL-1-L929) or human rhabdomyosarcoma (RD) cells displaying high SCARB2 but no PSGL-1 by treatment of specific inhibitors or siRNA. We found that disruption of clathrin-dependent endocytosis prevented EV71 infection in RD cells, while there was no influence in Jurkat T and PSGL-1-L929 cells. Disturbing caveolar endocytosis by specific inhibitor or caveolin-1 siRNA in Jurkat T and PSGL-1-L929 cells significantly blocked EV71 infection, whereas it had no effect on EV71 infection in RD cells. Confocal immunofluorescence demonstrated caveola, and EV71 was directly colocalized. pH-dependent endosomal acidification and intact membrane cholesterol were important for EV71 infection, as judged by the pretreatment of inhibitors that abrogated the infection. A receptor-dominated endocytosis of EV71 infection was observed: PSGL-1 initiates caveola-dependent endocytosis and hSCARB2 activates clathrin-dependent endocytosis.

Inhibition of enterovirus 71 replication and viral 3C protease by quercetin

BACKGROUND

Enterovirus 71 (EV71) is one of the major causative agents of hand, foot, and mouth disease (HFMD), which is sometimes associated with severe central nervous system disease in children. There is currently no specific medication for EV71 infection. Quercetin, one of the most widely distributed flavonoids in plants, has been demonstrated to inhibit various viral infections. However, investigation of the anti-EV71 mechanism has not been reported to date.

METHODS

The anti-EV71 activity of quercetin was evaluated by phenotype screening, determining the cytopathic effect (CPE) and EV71-induced cells apoptosis. The effects on EV71 replication were evaluated further by determining virus yield, viral RNA synthesis and protein expression, respectively. The mechanism of action against EV71 was determined from the effective stage and time-of-addition assays. The possible inhibitory functions of quercetin via viral 2Apro, 3Cpro or 3Dpol were tested. The interaction between EV71 3Cpro and quercetin was predicted and calculated by molecular docking.

RESULTS

Quercetin inhibited EV71-mediated cytopathogenic effects, reduced EV71 progeny yields, and prevented EV71-induced apoptosis with low cytotoxicity. Investigation of the underlying mechanism of action revealed that quercetin exhibited a preventive effect against EV71 infection and inhibited viral adsorption. Moreover, quercetin mediated its powerful therapeutic effects primarily by blocking the early post-attachment stage of viral infection. Further experiments demonstrated that quercetin potently inhibited the activity of the EV71 protease, 3Cpro, blocking viral replication, but not the activity of the protease, 2Apro, or the RNA polymerase, 3Dpol. Modeling of the molecular binding of the 3Cpro-quercetin complex revealed that quercetin was predicted to insert into the substrate-binding pocket of EV71 3Cpro, blocking substrate recognition and thereby inhibiting EV71 3Cpro activity.

CONCLUSIONS

Quercetin can effectively prevent EV71-induced cell injury with low toxicity to host cells. Quercetin may act in more than one way to deter viral infection, exhibiting some preventive and a powerful therapeutic effect against EV71. Further, quercetin potently inhibits EV71 3Cpro activity, thereby blocking EV71 replication.

A VP1 mutation acquired during an enterovirus 71 disseminated infection confers heparan sulfate binding ability and modulates ex vivo tropism

Enterovirus 71 (EV71) causes hand, foot and mouth disease, a mild and self-limited illness that is sometimes associated with severe neurological complications. EV71 neurotropic determinants remain ill-defined to date. We previously identified a mutation in the VP1 capsid protein (L97R) that was acquired over the course of a disseminated infection in an immunocompromised host. The mutation was absent in the respiratory tract but was present in the gut (as a mixed population) and in blood and cerebrospinal fluid (as a dominant species). In this study, we demonstrated that this mutation does not alter the dependence of EV71 on the human scavenger receptor class B2 (SCARB2), while it enables the virus to bind to the heparan sulfate (HS) attachment receptor and modifies viral tropism in cell lines and in respiratory, intestinal and neural tissues. Variants with VP197L or VP197R were able to replicate to high levels in intestinal and neural tissues and, to a lesser extent, in respiratory tissues, but their preferred entry site (from the luminal or basal tissue side) differed in respiratory and intestinal tissues and correlated with HS expression levels. These data account for the viral populations sequenced from the patient's respiratory and intestinal samples and suggest that improved dissemination, resulting from an acquired ability to bind HS, rather than specific neurotropism determinants, enabled the virus to reach and infect the central nervous system. Finally, we showed that iota-carrageenan, a highly sulfated polysaccharide, efficiently blocks the replication of HS-dependent variants in cells and 2D neural cultures. Overall, the results of this study emphasize the importance of HS binding in EV71 pathogenesis and open new avenues for the development of antiviral molecules that may prevent this virus's dissemination.

Amino Acid Variation at VP1-145 of Enterovirus 71 Determines Attachment Receptor Usage and Neurovirulence in Human Scavenger Receptor B2 Transgenic Mice

Infection by enterovirus 71 (EV71) is affected by cell surface receptors, including the human scavenger receptor B2 (hSCARB2), which are required for viral uncoating, and attachment receptors, such are heparan sulfate (HS), which bind virus but do not support uncoating. Amino acid residue 145 of the capsid protein VP1 affects viral binding to HS and virulence in mice. However, the contribution of this amino acid to pathogenicity in humans is not known. We produced EV71 having glycine (VP1-145G) or glutamic acid (VP1-145E) at position 145. VP1-145G, but not VP1-145E, enhanced viral infection in cell culture in an HS-dependent manner. However, VP1-145G virus showed an attenuated phenotype in wild-type suckling mice and in a transgenic mouse model expressing hSCARB2, while VP1-145E virus showed a virulent phenotype in both models. Thus, the HS-binding property and in vivo virulence are negatively correlated. Immunohistochemical analyses showed that HS is highly expressed in vascular endothelial cells and some other cell types where hSCARB2 is expressed at low or undetectable levels. VP1-145G virus bound to tissue homogenate of both hSCARB2 transgenic and nontransgenic mice in vitro, and the viral titer was reduced in the bloodstream immediately after intravenous inoculation. Furthermore, VP1-145G virus failed to disseminate well in the mouse organs. These data suggest that VP1-145G virus is adsorbed by attachment receptors such as HS during circulation in vivo, leading to abortive infection of HS-positive cells. This trapping effect is thought to be a major mechanism of attenuation of the VP1-145G virus.IMPORTANCE Attachment receptors expressed on the host cell surface are thought to enhance EV71 infection by increasing the chance of encountering true receptors. Although this has been confirmed using cell culture for some viruses, the importance of attachment receptors in vivo is unknown. This report provides an unexpected answer to this question. We demonstrated that the VP1-145G virus binds to HS and shows an attenuated phenotype in an hSCARB2-dependent animal infection model. HS is highly expressed in cells that express hSCARB2 at low or undetectable levels. Our data indicate that HS binding directs VP1-145G virus toward abortive infection and keeps virus away from hSCARB2-positive cells. Thus, although the ability of VP1-145G virus to use HS might be an advantage in replication in certain cultured cells, it becomes a serious disadvantage in replication in vivo This adsorption is thought to be a major mechanism of attenuation associated with attachment receptor usage.